1. Field of the Invention
The present invention relates generally to an orthogonal frequency division multiplexing (OFDM) communication system, and in particular, to an apparatus and method for transmitting and receiving side information in a partial transmit sequence (PTS) scheme.
2. Description of the Related Art
OFDM technology has high spectrum efficiency because spectra of sub-channels overlap with each other while maintaining orthogonality. According to the OFDM technology, input information symbols are modulated by inverse fast Fourier transform (hereinafter referred to as “IFFT”), while the IFFT-modulated signal is demodulated by fast Fourier transform (hereinafter referred to as “FFT”).
A brief description will now be made regarding operations of a transmitter and a receiver in a mobile communication system employing the OFDM technology (hereinafter referred to as “OFDM mobile communication system”).
In an OFDM transmitter, input data is modulated with a sub-carrier through a scrambler, a coder, and an interleaver. The transmitter provides a variable data rate, and applies a different coding rate, interleaving size, and modulation scheme according to the data rate. Commonly, the coder uses a coding rate of ½ or ¾, and the interleaving size for preventing a burst error is determined according to the number of coded bits per OFDM symbol (NCBPS). QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM (64-ary QAM) are used as the modulation scheme. A signal modulated with a predetermined number of sub-carriers by the elements stated above is summed up with a predetermined number of pilot sub-carriers, and then subjected to IFFT, thereby forming one OFDM signal. A guard interval for eliminating inter-symbol interference in a multi-path channel environment is inserted into the OFDM signal, and the guard interval-inserted OFDM signal is transmitted over a radio channel by a radio frequency (RF) processor, after passing through a symbol waveform generator.
In an OFDM receiver corresponding to the above-described transmitter, a reverse operation of the transmitter is performed and a synchronization process is added. First, a process of estimating a frequency offset and a symbol offset using a predetermined training symbol must be preferentially performed. Thereafter, a guard interval-eliminated data symbol is restored with a predetermined number of sub-carriers with which a predetermined number of pilot sub-carriers were summed up, through FFT. In order to cope with propagation delay in a radio channel environment, an equalizer eliminates signal distortion due to a channel from a received signal by estimating a channel condition. Data, a channel response of which was compensated through the equalizer, is converted into a bit stream and then deinterleaved by a deinterleaver. Thereafter, the deinterleaved data is restored into final data through a decoder for error correction and a descrambler.
The OFDM technology performs low-speed parallel transmission using a plurality of carriers instead of transmitting input data at high speed with a single carrier. That is, the OFDM technology is characterized in that it can realize a modulation/demodulation unit with an efficient digital device and is less susceptible to frequency selective fading or narrowband interference. Due to the characteristics stated above, OFDM technology is effective for the current European digital broadcasting transmission and high-speed data transmission adopted as the standard specification for a high-capacity mobile communication system, such as IEEE 802.11a, IEEE 802.16a, and IEEE 802.16b.
The OFDM mobile communication system transmits data with a plurality of sub-carriers, so an amplitude level of an OFDM signal can be represented by the sum of amplitude levels of the multiple sub-carriers. However, when a phase of each of the sub-carriers is changed without maintaining orthogonality, a phase of a particular sub-carrier may coincide with a phase of another sub-carrier. When phases of the sub-carriers are the same, an OFDM signal has a very high peak-to-average power ratio (hereinafter referred to as “PAPR”). An OFDM signal having a high PAPR reduces efficiency of a high-power linear amplifier and shifts an operating point of the high-power linear amplifier to a non-linear region, thereby causing inter-modulation distortion and spread spectrum between sub-carriers. In the OFDM communication system, the PAPR is a very important factor that affects communication performance. Therefore, a great amount of research has been conducted on a scheme for reducing the PAPR.
As a scheme for reducing PAPR in an OFDM communication system, there are provided a clipping scheme, a block coding scheme, and a phase rotation scheme. The clipping scheme, the block coding scheme, and the phase rotation scheme will be described herein below.
(1) Clipping Scheme
In the clipping scheme, if a level of a signal exceeds a predetermined threshold value, the level is forcibly clipped to the threshold value, thereby reducing PAPR. However, as a result, in-band distortion occurs due to non-linear operation, causing an increase in a bit error rate (hereinafter referred to as “BER”), and adjacent channel interference occurs due to out-band clipping noise.
(2) Block Coding Scheme
The block coding scheme codes surplus carriers and transmits the coded carriers to decrease PAPR of the whole carrier signal. The block coding scheme cannot only correct an error due to coding, but also decreases PAPR without distorting a signal. However, an increase in the number of the sub-carriers causes a drastic decrease in spectrum efficiency and an increase in the size of a look-up table or a generation matrix, disadvantageously increasing complexity and calculations.
(3) Phase Rotation Scheme
The phase rotation scheme is classified into a selective mapping (hereinafter referred to as “SLM”) scheme and a partial transmit sequence (hereinafter referred to as “PTS”) scheme. The SLM scheme multiplies each of M same data blocks by each of different phase sequences of length N, which are statistically independent of one another, selects a result having the lowest PAPR (i.e., a phase sequence having the lowest PAPR), and transmits the selected phase sequence. The SLM scheme is disadvantageous in that it requires M IFFT operations. However, the SLM scheme can considerably decrease PAPR and can be applied regardless of the number of sub-carriers.
Unlike the SLM scheme, the PTS scheme divides input data into M sub-blocks, performs L-point IFFT on each sub-block, multiplies each IFFT-transformed sub-block by a phase factor to minimize PAPR, and then sums the resultant sub-blocks before transmission. The PTS scheme is superior to the SLM scheme in terms of PAPR reduction, and is considered as the most effective and flexible scheme for reducing PAPR without non-linear distortion.
With reference to FIG. 1, a description will now be made of a transmitter for an OFDM communication system employing the PTS scheme (hereinafter referred to as “PTS-OFDM communication system”).
FIG. 1 illustrates an internal structure of a transmitter for a conventional OFDM communication system employing a PTS scheme. As illustrated in FIG. 1, a transmitter 100 for the PTS-OFDM communication system includes a mapper 110, a serial-to-parallel (S/P) converter 120, a sub-block segmentation unit 130, a plurality of IFFT units 140, 142, 144, and 146, a phase factor determiner 150, a plurality of multipliers 160, 162, 164, and 166, and a combiner 170.
Referring to FIG. 1, information bits to be transmitted are first coded at a predetermined coding rate, and coded bits generated by the coding are interleaved and then provided to the mapper 110 as input data X. Although various coding schemes have been proposed, a turbo coding scheme using a turbo code, which is an error correction code, is typically used as the coding scheme. The predetermined coding rate includes ½ and ¾.
The mapper 110 maps the input data X to a corresponding modulation symbol according to a predetermined modulation scheme, and the S/P converter 120 provides the modulation symbols sequentially output from the mapper 110 to L parallel lines, where L represents the number of taps of the IFFT units 140 to 146. The sub-block segmentation unit 130 segments modulation symbols output in parallel from the S/P converter 120 into M sub-blocks X(1) to X(M) having the same length N (L=N×M). It is assumed herein that the S/P converter 120 and the sub-block segmentation unit 130 are separately provided. Of course, however, the S/P converter 120 can be removed, and instead, the sub-block segmentation unit 130 can include a function of the S/P converter 120. In this case, the sub-block segmentation unit 130 segments L symbols sequentially provided from the mapper 110 into M sub-blocks having a length N.
A sub-block conversion operation of the sub-block segmentation unit 130 will be described with reference to FIGS. 2 to 4. FIG. 2 illustrates sub-blocks segmented according to an adjacent sub-block segmentation scheme, FIG. 3 illustrates sub-blocks segmented according to an interleaved sub-block segmentation scheme, and FIG. 4 illustrates sub-blocks segmented according to a pseudo random sub-block segmentation scheme. In all of the sub-block segmentation schemes, sub-blocks must be segmented so that each sub-block should not overlap with other sub-blocks.
A description will now be made of the sub-block segmentation schemes.
(1) Adjacent Sub-Block Segmentation Scheme
The adjacent sub-block segmentation scheme segments modulation symbols of length L into sub-blocks by the sequentially adjacent modulation symbols. As illustrated in FIG. 2, if the length L is 12, the adjacent sub-block segmentation scheme segments the modulation symbols of length 12 into 4 sub-blocks by the 3 sequentially adjacent modulation symbols.
(2) Interleaved Sub-Block Segmentation Scheme
The interleaved sub-block segmentation scheme segments modulation symbols of length L into sub-blocks by interleaving. As illustrated in FIG. 3, if the length L is 12, the interleaved sub-block segmentation scheme segments the modulation symbols of length 12 into a total of 4 sub-blocks by combining 3 modulation symbols at periods of 4 modulation symbols.
(3) Pseudo Random Sub-Block Segmentation Scheme
The pseudo random sub-block segmentation scheme segments modulation symbols of length L into sub-blocks by pseudo-randomly selecting the modulation symbols. As illustrated in FIG. 4, if the length L is 12, the pseudo random sub-block segmentation scheme segments the modulation symbols of length 12 into a total of 4 sub-blocks by randomly combining 3 modulation symbols without any rule or pattern.
In FIGS. 2 to 4, in each of sub-blocks segmented by the sub-block segmentation unit 130, all other symbols than the L symbols located in the determined positions are all replaced with 0.
The IFFT units 140 to 146 perform IFFT on each of the segmented sub-blocks and generate IFFT-transformed sub-blocks x(1) to x(M). The phase factor determiner 150 receives the IFFT-transformed sub-blocks x(1) to x(M) and determines phase factors {tilde over (b)}(1) to {tilde over (b)}(M) in such a way that phases of the sub-blocks should be different than one another, in order to minimize PAPR when the sub-blocks x(1) to x(M) are summed. Next, the phase factors are matched with their corresponding sub-blocks. That is, the phase factor {tilde over (b)}(1) is matched with the sub-block x(1). In this manner, the phase factor {tilde over (b)}(M) is matched with the sub-block x(M). The multipliers 160 to 166 multiply the IFFT-transformed sub-blocks x(1) to x(M) by the corresponding phase factors {tilde over (b)}(1) to {tilde over (b)}(M), and provide their outputs to the combiner 170. The combiner 170 generates an OFDM signal {tilde over (x)} by combining (or summing) the outputs of the multipliers 160 to 166.
As described above, the PTS scheme can effectively reduce PAPR without distorting a sub-channel spectrum and can be applied regardless of a digital modulation scheme. However, in order to enable a receiver to restore (or decode) information data, side information for a phase factor for phase rotation must be transmitted along with the data. Therefore, in order to realize the PTS scheme in an OFDM communication system, a method for effectively transmitting the side information is required.